1. Field of the Invention
The present invention is directed to a formation of a microcontact printing stamp employed in the creation of microcircuitry where dimensional integrity and registration must be maintained at the micron level over very large distances that may be as long as half a meter. More particularly, the present invention is directed to a process of preparing a high precision microcontact printing stamp in which printing stamp expansion is effected by gas exchange.
2. Background of the Prior Art
The process of microcontact printing, to create a very fine pitch pattern, is of recent vintage. This process is described in Kumar et al., Appl. Phys. Lett., 63, (14), 2002-2004 (October 1993) and Hidber et al., Langmuir, 12, 1375-1380 (1996). This process, which can conceivably replace photolithography in the fabrication of electronic components, especially where extremely fine line dimensions are required, requires the creation of a very fine pitch rubber stamp.
The very fine pitch rubber stamp utilized in microcontact printing is most often formed of an elastomeric material which is usually silicone rubber. Those skilled in the art are aware the term “silicone rubber” denotes polydimethylsiloxane (PDMS). In the current method of preparing rubber stamps used in high precision microcontact printing liquid, PDMS is introduced into a mold where a negative relief microcircuit pattern is expressed. The polymer is thereupon cured to produce a solidified rubber stamp which is removed from the mold. The solidified rubber stamp has a microcircuit pattern expressed in positive relief. It is this pattern that is transferred to a substrate in subsequent steps in the microcontact printing process.
The positive relief pattern provided on the rubber stamp is thereupon inked onto a substrate. Although there are several variations of microprinting methodology, commonly, the substrate is blanket coated with a thin gold film. The gold coated substrate is inked with an alkane thiol material transferred thereto by the stamp. Oftentimes, the alkane thiol material has the structural formula CH3—(CH2)18—CHSH2. It should be appreciated that other alkane thiol materials, as well as other inks, can be substituted for this alkane thiol.
Upon contact of the positive release pattern of the stamp with the gold film, a monolayer of the ink, preferably an alkane thiol, having the desired microcircuit pattern, is transferred to the gold film layer. Alkane thiols form an ordered monolayer on gold by a self assembly process. Thus, a self assembled monolayer (SAM) of the desired pattern is formed on the gold layer. The SAM is tightly packed and well adhered to the gold. As such, the SAM acts as an etch resist upon contact of a gold etching solution onto the stamped gold film layer.
In the next step, the inked substrate is immersed in a gold etching solution and all but the SAM is etched away to underlying layers below the gold layer. The SAM, which is unaffected by the gold etch, is then stripped away leaving gold in the desired pattern.
The aforementioned description is set forth in the Kumar et al. technical article. The Hidber et al. technical article utilizes a different procedure wherein the aforementioned rubber stamp is inked with a palladium catalyst and a pattern is again stamped onto a substrate. The positive relief microcircuit pattern of the palladium catalyst is subsequently immersed in an electroless plating solution which induces the desired microcircuit pattern by electroless plating.
The aforementioned description makes it apparent that faithful reproduction of the microcircuit pattern of the printing stamp onto the substrate is critical, especially when the pattern is of both fine pitch and of very large overall dimension. For example, if microcontact printing is used to produce microcircuitry on flat panel displays, it may require 5 micron sized defined features to accurately register to one another within one micron across a linear distance of 15 inches.
In turn, faithful reproduction of the microcircuit onto the substrate requires the fabrication of a microcircuit printing stamp that faithfully reproduces the desired microcircuit. This challenge to produce a high precision microcircuit printing stamp is magnified by the additional requirement that this formation of a microcircuit printing stamp be simple and cost effective. This latter requirement is emphasized because a primary application of this technology is the manufacture of flat panel displays. Flat panel displays must be produced at low cost and yet must meet the stringent tolerance criteria mentioned above.
In the past microcontact printing could not meet this challenge. This was because microcontact printing stamps could not satisfy the registration requirement because of shrinkage during printing stamp preparation. That is, the elastomeric polymer would shrink in the mold during printing stamp preparation. As those skilled in the art are aware, when an elastomeric polymer, such as silicone rubber, cures in a mold it shrinks to a degree of between about 0.1% to about 4%.
Thus, it is apparent that there is a strong need in the art for a new microcircuit printing stamp forming process that provides a stamp that provides good registration by compensating for the shrinkage that occurs during curing in the mold.